docs: correct StepResult field list in concepts guide

[sergiopaniego]

Summary

The Core Concepts guide listed StepResult fields that don't exist on the dataclass (terminated, truncated, info). Update the list to match the real fields: observation, reward, done, and
metadata (the latter added in #805).

Type of Change

• Documentation

Alignment Checklist

Before submitting, verify:

• I have read .claude/docs/PRINCIPLES.md and this PR aligns with our principles
• I have checked .claude/docs/INVARIANTS.md and no invariants are violated
• I have run /pre-submit-pr (or bash .claude/hooks/lint.sh and tests) and addressed all issues

RFC Status

• Not required (bug fix, docs, minor refactoring)

Test Plan

Docs-only change. Verify the StepResult bullet list in docs/source/guides/concepts.md matches the dataclass in src/openenv/core/client_types.py (observation, reward, done, metadata).

Claude Code Review

N/A

[bot-ci-comment]

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